1. Field of the Invention
This invention relates to an omnidirectional one-dimensional photonic crystal and a light emitting device made from the same.
2. Description of the Related Art
U.S. Pat. No. 5,813,753 discloses a light emitting device that includes a UV/blue LED located in a depression having reflecting sidewalls, a light transmitting material surrounding the LED and filling the depression, a phosphor material in the form of particles dispersed in the light transmitting material, and a long-wave pass (LWP) filter formed on a front side of the light transmitting material.
FIG. 1 illustrates a conventional light emitting device 10 that is disclosed in U.S. Pat. No. 6,155,699, and that includes a cup 11 defining a depression 12, a light emitting diode 13 placed in the depression 12, a dome-shaped encapsulating layer 14 encapsulating the light emitting diode 13, a Distributed Bragg Reflector (DBR) mirror 15 surrounding the encapsulating layer 14, a wavelength-converting member 16 surrounding the DBR mirror 15, and a lens 17 encapsulating the wavelength-converting member 16. The DBR mirror 15 is known in the art as a multi-layered dielectric structure having a spatially periodic variation in dielectric constant and exhibiting a frequency photonic bandgap characteristic that prevents propagation of light in a certain frequency range within the dielectric structure and that permits total reflection of the light. The wavelength-converting member 16 is normally made from phosphorescent materials, which are known in the art as an agent for absorbing and converting a primary light (e.g., an invisible or UV/blue light), which has a shorter wavelength range, into a secondary light (e.g., a visible or white light), which has a longer wavelength range. The DBR mirror 15 possesses a transmittance characteristic of transmitting most of the first light therethrough and to the wavelength-converting member 16, and a reflectance characteristic of preventing the second light generated from the wavelength-converting member 16 from entering to the encapsulating layer 14. In use, the light emitting diode 13 emits a primary light that passes through the encapsulating layer 14 and the DBR mirror 15, and that is subsequently converted into a secondary light by the phosphorescent material in the wavelength-converting member 16. A portion of the secondary light exits the light emitting device 10 through the lens 17, while the remainder of the secondary light impinges the DBR mirror 15 and is subsequently reflected by the latter back to the wavelength-converting member 16 so as to prevent the secondary light from entering the encapsulating layer 14, thereby enhancing the efficiency of the light emitting device 10.
Since the amount of the primary light converted into the secondary light depends on the concentration and the quantum efficiency of the phosphorescent materials in the wavelength-converting member 16, a significant amount of the primary light may not be converted and may pass through the wavelength-converting member 16 and the lens 17 and into the air, which results in a decrease in the efficiency of the light emitting device 10 and in the quality of the secondary light, such as color temperature and purity, and which can be harmful to the environment if the primary light is a UV light. Therefore, there is a need for improving efficiency of converting the primary light into the secondary light so as to enhance the efficiency of the light emitting device 10.
The aforesaid DBR mirror and the LWP filter are dielectric structures with pairs of high and low refractive index layers. It is known that the conventional DBR mirrors and the LWP filters do not work so well to reflect or transmit light over a wide range of incident angles relative to a normal line of a surface of the dielectric structure of the DBR mirror or the LWP filter.
U.S. Pat. No. 6,130,780 discloses an omnidirectional reflector that is made from an omnidirectional one-dimensional photonic crystal possessing omnidirectional photonic bandgaps and that is capable of totally reflecting the light with any incident angle and polarization when the frequency (or wavelength) of the incident light falls in said bandgaps. The disclosed reflector consists of pairs of high and low refractive index layers. The reflective index contrast between the two dielectric materials should be high enough to form omnidirectional photonic bandgaps.
The entire disclosures of U.S. Pat. Nos. 6, 155,699, 5,813,753, and 6,130,780 are hereby incorporated herein by reference.